MRAMs have been widely investigated and used as nonvolatile memory devices that can be operated at low voltage and at high speed. In an MRAM cell, data is stored in a magnetic resistor that includes a Magnetic Tunnel Junction (MTJ) having first and second ferromagnetic layers and a tunneling barrier layer therebetween. In some devices, the magnetic polarization of the first ferromagnetic layer, also referred to as a free layer, is changed utilizing a magnetic field that crosses the MTJ. The magnetic field may be induced by an electric current passing around the MTJ, and the magnetic polarization of the free layer can be parallel or anti-parallel to the magnetic polarization of the second ferromagnetic layer, also referred to as a fixed layer. According to spintronics based on quantum mechanics, a tunneling current passing through the MTJ in the parallel direction may be greater than that in the anti-parallel direction. Thus, the magnetic polarizations of the free layer and the fixed layer can define the electrical resistance of the magnetic resistor, to provide an indication of the stored information in the MRAM.
The MJT is generally formed on a lower electrode of stoichiometric TiN consisting of a 1:1 mixture of titanium and nitrogen. Chemical Mechanical Polishing (CMP) is then performed on the stoichiometric TiN to obtain a desired thickness thereof. It is well known that CMP speed of stoichiometric TiN may be high, such as a rate of about 40 Å/sec. Accordingly, there may be difficulty in controlling a desired thickness of residual TiN after CMP. In other words, the lower electrode of TiN may not be formed at a desired thickness. For example, even if the lower electrode is desired to be 400 Å to 500 Å, an initial deposition thickness of TiN should be about 1,000 Å at a minimum in consideration of the thickness removed during CMP. Therefore, process costs may increase.
Furthermore, since stoichiometric TiN has a relatively high CMP speed, the surface roughness of stoichiometric TiN may be excessive after CMP. Surface roughness of an aluminum layer formed on the TiN also may be excessive. Aluminum performs a function to form tunneling barrier of the MTJ that will be deposited in a subsequent process. Accordingly, inadequate magnetic resistance ratio (MR) and/or resistance (RA) of the MJT may occur.
In another approach, crystalline stoichiometric TiN is used as a lower electrode. However, if the MTJ is deposited on an upper surface of stoichiometric TiN, several layers or films of the MTJ may depend on the crystallinity of the crystalline stoichiometric TiN lower electrode.
Specifically, when an aluminum oxide layer (AlOX) as a tunneling barrier is orientated in a (111) plane, the tunneling effect of current may be increased. Because aluminum is a face centered cube (FCC), the most closely packed plane density of aluminum exhibits a (111) plane. Accordingly, with improving (111) orientation, aluminum is formed to be closely packed structure. Also, an aluminum oxide layer (AlOX) that will be oxidized in a subsequent process may be formed with high density to increase magnetic resistance ratio (MR).
However, since the orientation of aluminum may depend on the orientation of the stoichiometric TiN used as the lower electrode, it may be difficult for aluminum to be formed with the most closely packed structure. As a result, it may be difficult to accomplish high magnetic resistance ratio (MR) and/or tunneling effect with high current.